1. Field of the Invention
This invention relates to a method of making a phosphor containing glass plate and to a method of making a light emitting device by using the phosphor containing glass plate.
2. Description of the Related Art
In recent years, white LED (light emitting diode) lamps for emitting white light are practically used as a lighting system, a backlight for LCD (liquid crystal display) etc.
In general, the white LED lamps are composed of a blue LED chip which is made of a group III nitride-based compound semiconductor for emitting blue light, and a transparent resin which is made of epoxy system, silicone system etc. for sealing the LED chip and contains a yellow phosphor powder that emits yellow wavelength-converted light by being excited by light emitted from the LED chip.
In order to have a high color rendering property than the above white LED lamps, a white LED lamp has been proposed that seals the LED chip with the transparent resin which contains green and red phosphor powders as well as the yellow phosphor powder.
For example, related arts to the invention are as follows.
JP-A-2006-253336
JP-A-2002-203989
JP-A-2003-258308
However, since the conventional white LED lamps use the transparent resin as the sealing material, the transparent resin near the LED chip may be yellowed due to light or heat etc. from the LED chip, so that the light extraction efficiency will deteriorate with time. In particular, the yellowing of the transparent resin is notably caused when a short-wavelength LED chip for emitting blue light etc. with high energy.
Further, since the transparent resin has permeability to gases, the phosphor powder contained in the transparent resin may be reacted with gas (especially moisture) in the air, so that the phosphor characteristics can often deteriorate with time. The deterioration of characteristics is notably caused in a sulfide phosphor, an aluminate phosphor and a silicate phosphor which are easy hydrolyzed and low in moisture resistance. Because of this, it is difficult to keep the characteristics of these phosphors although they can be used as high excitation efficiency green or red phosphor.
As described, sealing by the transparent resin is insufficient to meet the heat resistance, the light resistance and the impermeability to gases which are demanded for LED lamps.
In order to solve the problems caused by the transparent resin, a glass-sealed LED lamp has been proposed in which an LED chip is sealed with glass to meet the heat resistance, the light resistance and the impermeability to gases not provided for the transparent resin (See, e.g., JP-A-2006-253336 and JP-A-2002-203989).
In order to yield white light using the glass-sealed LED lamp, a method may be generally envisaged that phosphor powder is mixed into the glass as done in the transparent resin. In this case, it is desirable that the phosphor is uniformly dispersed so as to prevent unevenness in emission color of the LED lamp. However, since glass has a high viscosity even at a temperature (i.e., yielding point (At)) where it starts to be softened, it is exactly very difficult to uniformly disperse the phosphor therein. To have a low viscosity to uniformly disperse the phosphor in the glass, it is necessary to heat it to about 1000° C. However, at such a high temperature, the phosphor may be reacted with the glass so that the phosphor loses the fluorescence characteristic.
A method for uniformly dispersing the phosphor in glass is suggested in which glass powder and inorganic phosphor powder are mixed into a resin binder, a provisional molded piece with a desired shape is produce by press molding, the provisional molded piece is baked to remove the resin binder to form a phosphor-containing glass (See JP-A-2003-258308). However, in this method, the glass characteristic such as strength and transparency may deteriorate due to the remainder of the resin binder. Further, due to the vaporization of the remainder during the baking, air bubbles may be generated in the glass to allow gas permeability. Therefore, phosphors with low moisture resistance will be subject to deterioration in fluorescence characteristic.
Another method for yielding white light may be envisaged in which phosphor powder is mixed in a sol-gel glass precursor (e.g., solution of solvent:alcohol and solute:metal alkoxide) which can be formed at a relatively low temperature. However, since the sol-gel glass is porous and therefore has gas permeability, phosphors with low moisture resistance will be subject to deterioration in fluorescence characteristic. Further, since the sol-gel glass is large in volume change before and after the vitrification, it may cause cracks when it is formed into a thick film. Therefore, it is difficult to use it as a sealing material which is needed to be formed relatively thick.